1. Field of the Invention
The present invention relates to a helmet with a built-in speaker system and a speaker system for a helmet, and more particularly relates to a helmet with piezoelectric film speakers, and piezoelectric film speakers for a helmet.
2. Description of the Background Art
Magnetic speaker systems including a voice coil mounted on a cone have been widely used in helmets up to this time. However, such speaker systems are difficult to downsize or make light in weight. There has been a great demand for improved speaker systems for helmets. In order to attach a speaker system in the confined space of a helmet and make a user feel comfortable, the use of the piezoelectric film speakers has been proposed in Japanese Patent Laid-Open No. Sho 63-175106 and Japanese Utility Model Laid-Open No. Sho 63-44584.
The piezoelectric film expands and contracts in response to voltages applied to an electrode. Therefore, when applying the piezoelectric film to a speaker, it is necessary to convert expansion and contraction of the surface of the piezoelectric film into oscillations of the film surface. This requirement is satisfied by maintaining the piezoelectric film in a curved state, which enables the piezoelectric film to function as a speaker.
In such a case, output performance in response to input signals, i.e. sound pressure level and frequency characteristics of the piezoelectric film as a speaker, depends upon the relative relationship between the expansion-contraction, and the oscillation. This relative relationship depends upon the degree to which the piezoelectric film is curved, i.e. a radius of curvature of the curved portion of the piezoelectric film. Further, there is a limitation to the radius of curvature in order for which the piezoelectric film to perform in the required manner. If such a limitation is not observed and is exceeded, the piezoelectric film speaker cannot produce a sufficient sound pressure level or may have deteriorating frequency characteristics, so that the speaker cannot demonstrate the desired capacity and sound quality.
In the foregoing related art, no definite measures seem to have been taken in order to strictly keep the piezoelectric film curved in the predetermined range. Therefore, existing piezoelectric film speakers do not function satisfactorily.